A local-area network ("LAN") is a communication system that enables a group of communication stations located within a limited geographical area such as an office, a building, or a cluster of buildings to electronically transfer information among one another. One form of communication station is data terminal equipment ("DTE"), typically a personal computer or a work station. DTE originates messages and/or constitutes the ultimate destination for messages. DTE also provides communication control functions. Other forms of communication stations are repeaters (or hubs), file servers, and bridges.
Information transferred between communication stations in a LAN can be generally placed in the categories of (a) data, (b) control information relating to start/stop of data transmission, and (c) other control information. As used here with respect to a communication station in a LAN, the term "in-band" generally refers to periods in which the station is transmitting packets of data and control information relating to start/stop of data transmission to another communication station in the LAN. The term "out-of-band" is basically the converse of "in-band". As used here with respect to a communication station in a LAN, "out-of-band" thus generally refers to periods when the station is not transmitting packets of data and control information relating to start/stop of data transmission. During out-of-band periods, the communication station may send other types of control information, such as configuration or linkage information, to another communication station in the LAN.
Communication stations in a LAN exchange data and control information with one another by following a fixed protocol which defines the network operation. The ISO Open Systems Interconnection Basic Reference Model establishes a seven-layer LAN communication model. The two lowest layers in the model are the physical layer and the data link layer. The physical layer consists of modules that specify (a) the physical media which interconnects the communication stations and over which information is to be electronically transmitted, (b) the manner in which the communication stations interface to the physical transmission media, (c) the process for transferring information over the physical media, and (d) the protocol of the information stream. The data link layer includes a logical link control sublayer and a media access control ("MAC") sublayer that interfaces with the physical layer directly or by way of a reconciliation sublayer.
IEEE Standard 802.3, Carrier Sense Multiple Access with Collision Detection ("CSMA/CD") Access Method and Physical Layer Specifications, is one of the most widely used standards for the physical layer and the MAC sublayer. Commonly referred to as Ethernet, IEEE Standard 802.3 prescribes various rates for transferring data.
The 10Base-T protocol of IEEE Standard 802.3 deals with transferring data at a rate of 10 megabits/second ("Mbps") over twisted-pair copper cables. Consider a LAN containing two communication stations that can transfer data only at the 10-Mbps rate of the 10Base-T protocol. Before one of the stations starts transmitting data to the other, the intended transmitting station first establishes that there is a 10Base-T communication link with the intended receiving station. This is accomplished with link pulses that each station transmits during out-of-band periods directly after power-up. The link pulses, commonly termed "normal" link pulses, consist of 100-ns pulses provided every 16 ms.+-.8 ms. When the intended transmitting station receives a sufficient number of normal link pulses to indicate the presence of a link to a communication station capable of receiving data at the 10Base-T rate, the transmitting station begins sending data.
A protocol referred to as 100Base-TX is under consideration for expanding IEEE Standard 802.3 to accommodate data moving at an effective rate of 100 Mbps through twisted-pair copper cables of presently existing types. The 100Base-TX protocol leverages on the ANSI X3T12 standard, generally termed FDDI for fiber data distributed interface, which covers the transmission of data at 100 Mbps over fiber optical cables. In fact, the proposed expansion of IEEE Standard 802.3 includes a protocol termed 100Base-FX for sending data over fiber optical cables at an effective rate of 100 Mbps. For matters common to 100Base-TX and 100Base-FX, the two protocols are known as 100Base-X.
Under the proposed 100Base-X protocol, certain control information is incorporated into a 100Base-X data stream before it is placed on a copper or fiber optical cable. In particular, the MAC sublayer in the transmitting station supplies data in 4-bit code groups often referred to as nibbles. The physical layer in the transmitting station contains a physical coding sublayer ("PCS") that converts the 4-bit code groups into 5-bit code groups often referred to as symbols. Each 5-bit code group has the same total bit duration, approximately 40 ns, as a 4-bit code group. The 4-bit/5-bit conversion performed in the PCS increases the number of available code groups. This provides a capacity for incorporating control information into the data stream. After scrambling, serialization, and additional coding to reduce electromagnetic interference, the resulting coded information moves at 125 Mbps on the cable. The 100Base-X mapping between the 4-bit MAC data code groups and the 5-bit PCS code groups is given in the following table:
TABLE 1 ______________________________________ 5-bit PCS 4-bit MAC Code Group Code Group Name or Meaning ______________________________________ 11110 0 0000 01001 1 0001 10100 2 0010 10101 3 0011 01010 4 0100 01011 5 0101 01110 6 0110 01111 7 0111 10010 8 1000 10011 9 1001 10110 A 1010 10111 B 1011 11010 C 1100 11011 D 1101 11100 E 1110 11101 F 1111 11111 I Idle 11000 J First SSD code group, used with K 10001 K Second SSD code group, used with J 01101 T First ESD code group, used with R 00111 R Second ESD code group, used with T 00100 H Indicates transmit error 00000 V Invalid 00001 V Invalid 00010 V Invalid 00011 V Invalid 00101 V Invalid 00110 V Invalid 01000 V Invalid 01100 V Invalid 10000 V Invalid 11001 V Invalid ______________________________________
Half of the 5-bit code groups correspond to the 4-bit code groups. Part of the other half of the 5-bit code groups are used for control purposes. The remainder of the other half of the 5-bit code groups are not utilized and, therefore, are labeled as invalid in Table 1. The acronyms "SSD" and "ESD" in Table 1 respectively mean start-of-stream delimiter and end-of-stream delimiter.
Referring to the drawings, FIG. 1 illustrates the MAC-to-PCS 4-bit/5-bit conversion in more detail. Data from the MAC sublayer is provided during in-band periods referred to as frames. Each MAC frame consists of a preamble, a start-of-frame delimiter ("SFD"), and a data section. The preamble is formed with up to seven preamble octets, each consisting of eight bits--i.e., a pair of 4-bit code groups. The start-of-frame delimiter takes up one octet. The data section contains 46-1500 pairs of 4-bit code groups. Each bit is a binary "0" or a binary "1".
A pair of out-of-band periods referred to as inter-frame gaps enclose each MAC frame. The acronym "IFG" in FIG. 1 means inter-frame gap. During the inter-frame gaps, the MAC sublayer supplies no information. The bits in each MAC inter-frame gap thus are "0s".
In converting the MAC data into the 100Base-X PCS stream of 5-bit code groups, the 10-bit SSD code-group pair JK is substituted for the first preamble octet (i.e., the first two 4-bit preamble code groups) in the MAC frame so as to indicate the start of the 100Base-X PCS stream. Each pair of 4-bit MAC data code groups is converted into a corresponding pair of 5-bit 100Base-X code groups according to Table 1. At the end of the MAC frame, the PCS appends the 10-bit ESD code-group pair TR to indicate the end of the 100Base-X PCS stream.
The portion of the 100Base-X inter-frame gap following end-of-stream delimiter TR constitutes the out-of-band period for the physical layer. During this part of the inter-frame gap, the PCS furnishes the idle code group I to indicate the presence of a good communication link. As indicated in Table 1, each I code group consists of five "1s". A mapping opposite to that described above occurs at the communication station when it receives 100Base-X data from another communication station.
The physical layer in one communication station asserts a carrier-sense signal whenever the physical layer receives 100Base-X data from another communication station. In particular, carrier sense is asserted when a pair of non-contiguous "0s" are detected within any 10-bit portion of the overall stream of 5-bit code groups coming into the PCS during data reception. A pair of "0s" are "non-contiguous" within a 10-bit stream segment when two "0s" are separated by at least one other code bit. For example, the 10-bit segments (0101111111) and (1111111000) both contain a pair of non-contiguous "0s", whereas the 10-bit segment (1111001111) does not contain a pair of non-contiguous "0s". Carrier sense is de-asserted when the ESD signal pair TR is detected and also when ten contiguous "1s" such as the signal pair II, are detected in the overall 100Base-X incoming bit stream.
As with the 10Base-T protocol, a prerequisite for enabling one communication station to transmit data to another in accordance with the 100Base-X protocol is for the transmitting station to establish a 100Base-X communication link with the receiving station. This involves initially determining whether the receiving station can receive (i.e., properly process) 100Base-X data and, if so, periodically verifying that the receiving station remains capable of receiving 100Base-X data.
In specifying how a 100Base-X communication link is to be set up when the transmission medium consists of twisted-pair copper cables, the proposed extension of IEEE Standard 802.3 to include the 100Base-X protocol establishes the NWay autodetect process to take into account the fact that two communication stations which meet IEEE Standard 802.3 may be able to communicate solely according to 100Base-TX, solely according to 10Base-T, according to either 10Base-T or 100Base-TX, according to another protocol such as 100Base-T4, or according to none of these protocols. See "MAC Parameters, Physical Layer, Medium Attachment Units and Repeater for 100 Mb/s Operation (version 1.0)," CSMA/CD Access Method & Physical Layer Specifications, Draft Supplement to 1993 version of ANSI/IEEE Document #802.3u/d2, Std 802.3, Chapter 28, 24 Jul. 1994. Also see "IEEE Link Task Force Autodetect", Specification for NWay Autodetect, National Semiconductor, Version 1.0, 10 Apr. 1994.
For example, one communication station may only have 100Base-TX capability, while another communication station can operate at 100Base-TX and 10Base-T. Data is then transmitted according to the 100Base-TX protocol. Alternatively, each communication station may be capable of communicating at both 100Base-TX and 10Base-T. Although the two stations could theoretically communicate according to either of these two protocols, 100Base-TX is preferable because it is much faster. Finally, one station may solely utilize 10Base-T while another station solely utilizes 100Base-TX so that the stations cannot communicate directly with each other.
Under the NWay autodetect procedure, a communication station contains a link negotiator which produces a burst of "fast" link pulses that carry information specifying the station's processing capability. The fast link pulses indicate whether the station operates in 10Base-T, 100Base-TX, or 100Base-T4 mode. The fast link pulses also indicate whether the station can simultaneously transmit and receive data (full duplex) or can do only one of transmitting and receiving data at a time (half duplex). The control information contained in the fast link pulses is placed in certain ones of thirty-two 16-bit management control registers contained in the PCS.
Two communication stations that are provided with NWay link negotiators exchange fast link pulses until each station determines that the other is applying the NWay procedure. A communication link is then set up. Data transmission subsequently occurs at the highest common denominator of data transmission capability. For example, if one station operates at 100Base-TX in full duplex while the other can operate either at 10Base-T or 100Base-TX in half duplex, data transmission occurs at the 100Base-TX data rate in half duplex. If there is no common denominator of data transmission capability, neither station transmits data to the other.
Each burst of fast link pulses contains 16 bits of information. The fast-link pulse bursts are provided at the same frequency as the normal 10Base-T link pulses. That is, the spacing between the beginnings of the fast link pulse bursts is typically 16 ms. Accordingly, the average bit transfer rate for the fast-link pulse bursts typically is only 1 kilobit/second.
The NWay autodetect procedure is a useful technique for establishing the optimal mode by which two communication stations can exchange linkage and capability information in accordance with IEEE Standard 802.3. However, the bit rate for the fast link pulses is tied to the 10Base-T protocol and thus is quite low for the 100Base-X protocol in which data moves approximately ten times faster. It would be highly desirable to have an out-of-band signalling method for transferring station status information, including linkage information, at a considerably faster rate than in current NWay-based 100Base-X LAN applications without causing carrier sense to be falsely asserted and without introducing information fragments that may clutter the communication network.